Cthonians are subterranean creatures that have been described as giant worms, or earth-bound squid. They have long, featureless bodies that taper on one end, while the other sports a number of powerful tentacles. They burrow through rock as if it were water, and travel comfortably deep below its surface.

One of their known powers is mental control. They are able to compel humans to do certain things, though they tend no to be complex operations. Usually they root the human to a particular spot to make them better targets.

They are also able to create earthquakes. The size of the earthquake depends on the size of the Cthonian. Cthonians can also work together to form larger quakes.

Cthonians are associated with a strange city in Africa called G'harne, whose location is a mystery.

There is one Cthonian called Shudde M'ell who is described as the leader of the Cthonians. What influence he has over other Cthonians is not known.